1. Field of the Invention
The present invention relates to a hydraulic circuit having a holding valve of an external pilot pressure operation type, which can prevent or reduce a push phenomenon of an actuator (e.g. hydraulic cylinder and so on) due to leakage of hydraulic fluid of a main control valve that controls the hydraulic fluid being fed to the actuator.
More particularly, the present invention relates to a hydraulic circuit having a holding valve of an external pilot pressure operation type, which can make a double check valve, which is installed to prevent an actuator from being pushed due to leakage of hydraulic fluid of a main control valve, be opened by an external signal pressure being supplied to shift the main control valve.
2. Description of the Prior Art
As illustrated in FIG. 1, a conventional hydraulic circuit includes a hydraulic pump p connected to and engine (not illustrated) and a pilot pump Pp; a hydraulic cylinder 1 connected to the hydraulic pump p to operate a working device such as a boom; a main control valve 2 installed in a flow path between the hydraulic pump p and the hydraulic cylinder 1 and shifted to control a start, a stop, and a direction change of the hydraulic cylinder 1 in response to a pilot signal pressure being supplied from the pilot pump Pp; and a joystick 11 supplying the pilot signal pressure from the pilot pump Pp to the main control valve 2 in accordance with an amount of user's manipulation thereof.
In the drawing, the reference numeral “R” denotes a relief valve installed in a flow path of the pilot pump Pp to protect a hydraulic system by draining a part of the hydraulic fluid to a hydraulic tank if overload that exceeds a pressure set in a pilot signal line occurs.
When the hydraulic cylinder 1 is stopped, hydraulic fluid leaks through a spool gap of the main control valve 2 (i.e. leakage of hydraulic fluid occurs through a gap of a spool installed in the main control valve 2 to be shifted in accordance with the pilot signal pressure being applied from an outside to the main control valve 2), and thus the hydraulic cylinder 1 is pushed to cause a safety accident against a user's intention to occur.
As illustrated in FIG. 2, another conventional hydraulic circuit having a holding valve of an internal pilot signal pressure operation type includes a hydraulic pump p and a pilot pump Pp respectively connected to an engine (not illustrated); a hydraulic cylinder 1 connected to the hydraulic pump p to operate a working device such as a boom; a main control valve 2 installed in a flow path between the hydraulic pump p and the hydraulic cylinder 1 and shifted to control a start, a stop, and a direction change of the hydraulic cylinder 1 in response to a pilot signal pressure being supplied from an outside; a joystick 11 supplying the pilot signal pressure from the pilot pump Pp to the main control valve 2 in accordance with an amount of user's manipulation thereof; and a double check valve 3 (composed of a check valve 4 and a valve spring 5 installed in the flow path, respectively, to serve as a holding valve) installed in a flow path between the main control valve 2 and the hydraulic cylinder 1 to be opened/closed by hydraulic pressure being supplied to the hydraulic cylinder 1, and preventing the hydraulic cylinder from being pushed due to leakage of hydraulic fluid through a spool gap of the main control valve 2.
At this time, a spool of the main control valve 2 is formed to be connected to a hydraulic tank when it is in a neutral state so as to prevent the check valve 4 from being opened due to a back pressure of the main control valve.
If a resilient force of the valve spring 5 is larger than a preset value, the check valve 4 is easily opened by the hydraulic pressure being supplied to the hydraulic cylinder 1, and thus a large amount of hydraulic fluid is discharged. Accordingly, if the back pressure of the main control valve 2 is increased, the check valve 4 is opened to cause a push phenomenon of the hydraulic cylinder 1 to occur.
By contrast, if the resilient force of the valve spring 5 is smaller than the preset value, the check valve 4 is opened after the hydraulic pressure being supplied to the hydraulic cylinder 1 reaches a predetermined pressure value. Accordingly, it is difficult to finely manipulate the working device to lower the manipulation, and this may cause an abrupt operation of the hydraulic cylinder 1.
Also, the check valve 4 repeats an instantaneous opening/closing operation under specified work conditions, and thus a shaking phenomenon occurs to cause severe vibration and noise.